Final answer:
The correct statement according to Le Châtelier's principle is a: Addition of a substance to the left side of an equilibrium shifts it to the right as the system adjusts to the added concentration of reactants by forming more products. (Option A).
Step-by-step explanation:
When applying Le Châtelier's principle, the correct statement that describes how a chemical system at equilibrium responds to a disturbance is option a: Addition of a substance to the left side of an equilibrium shifts it to the right.
This reflects the principle's assertion that if a stress in the form of increased concentration of reactants (on the left side) is applied to the system at equilibrium, the system adjusts to relieve that stress by shifting the reaction to form more products, and thus shifts to the right. Options b and c are incorrect because any stress to a system at equilibrium will cause a shift to either side to relieve the stress depending on the direction of the reaction and the nature of the stress.
Furthermore, Le Châtelier's principle indicates that changes in temperature can shift the equilibrium depending on whether the forward reaction is exothermic or endothermic. If heat is added to an exothermic reaction, the system will shift to the left, thus opposing the addition (since the reaction releases heat), making option c incorrect. In summary, for option d, both responses b and c are incorrect applications of Le Châtelier's principle. (Option A).